Light-emitting devices using LED chips (LED devices) are finding wide applications because of demand for light-emitting devices with higher luminance and lower energy consumption. In particular, white LED devices are increasingly used as lighting sources such as electric lights, which require white light, and backlights for liquid crystal displays in the field of lighting devices.
An example of the white LED devices is a device that use a combination of a blue LED chip and a phosphor that emits yellow light upon receipt of blue light. Other examples of the white LED device include white LED devices that create white light by a combination of a LED chip that emits UV light and phosphors that emit, upon receipt of UV light, blue, green and red lights; and white LED devices that create white light by a combination of a LED chip that emits blue light and phosphors that emit red and green lights. In the above-mentioned white LED devices, the light emitted by the LED chip and the fluorescence emitted by the phosphor are mixed, thereby obtaining white light.
The white LED devices that use a combination of an LED chip and a phosphor can generate white light with a single light source (LED chip). Thus, as compared to white LED devices that use a combination of multiple LED chips of different colors for generation of white light, the device can be more simplified and power consumption can be reduced.
However, the light from white LED devices using a combination of an LED chip and a phosphor is colored when a balance between the emission light from the LED chip and fluorescence from the phosphor is disrupted. The disrupted balance also causes “color non-uniformity,” a phenomenon where color (chromaticity) varies depending on the observation angle of the device.
One cause of light coloring and color non-uniformity pertinent in the white LED device is the non-uniform distribution of phosphors in the LED device. Generally, phosphors are dispersed in resin. Phosphor, however, is generally an inorganometallic compound having an extremely high specific gravity. Therefore, when liquid resin containing dispersed phosphor particles is applied, the phosphor particles settle down, resulting in that the phosphor particles are unevenly deposited.
One disclosed technique for reduced color non-uniformity of light emitted from the white LED device involves adding an anti-settling agent of phosphor particles to liquid resin thus preventing settling of the phosphor particles (see PTL 1). In PTL 1, application of the liquid resin containing the phosphor particles is followed by curing while spinning, thereby limiting settlement of the phosphor particles (see PTL 1).
In recent years, dispersion of phosphor particles to light transmissive ceramic (glass) has been studied. In this method, however, it is difficult to apply the dispersion liquid on the corners and lateral surfaces of the LED chip since the viscosity of the dispersion liquid containing dispersed phosphor particles is low. As a result, the wavelength conversion efficiency is easily degraded, and chromaticity deviation and color non-uniformity easily occur among different directions. Under such circumstances, a method has been proposed in which color non-uniformity is reduced by spraying a spiral of a coating solution containing a phosphor and a ceramic precursor (see PTL 2).